1. Field of the Invention
The instant invention relates generally to variable displacement pumps and more specifically to a horsepower limiter control which automatically limits the maximum pump horsepower to a set amount by adjusting the product of maximum pump working pressure and pump displacement to maintain a constant horsepower.
2. Description of the Prior Art
One prior control for limiting pump horsepower to a preset value, known to applicants, operates by automatically changing the setting of a working pressure sequence or compensator valve as the displacement changes. This control utilizes a pilot flow from working pressure fluid which flows through an adjustable fixed orifice in the sequence valve which orifice sets the pump horsepower limit and provides a constant fluid flow through a variable power limiter control orifice. The power limiter control orifice is downstream of the fixed orifice and varies in size in relation to the displacement setting of the pump. This causes the pressure of the fluid between the two orifices, which fluid acts on the sequence valve remote control connection, to change with the displacement setting. Thus, the setting of the sequence valve varies as the displacement of the pump is changed.
When the pressure and flow of the pump become such that the horsepower set by the fixed orifice is exceeded, the horsepower limiter operates and the sequence valve is spilled. Simultaneously, working pressure fluid is directed to a fluid motor which automatically destrokes the pump until its horsepower is decreased to that set by the fixed orifice.
This control requires a uniform fluid flow from the sequence valve remote control line through the horsepower limiter control orifice during operation of the horsepower limiter, i.e. when the pump horsepower tends to be excessive and the displacement of the pump is automatically reduced, in order to provide an accurate horsepower setting for the valve. Such a uniform flow is difficult to achieve because of tolerances in the dimensions of the sequence valve poppet and seat, because of friction and because of leaks in the valve.
A further disadvantage of the prior system is that the variable horsepower limiter orifice, which is controlled by the position of the rocker cam, i.e. pump displacement, is necessarily extremely small since high pressure working fluid is utilized as the control medium. In this control the contour of the orifice must be precisely manufactured to maintain a constant set horsepower. Since the orifice is very small, it is difficult to precisely manufacture this orifice. Further, the orifice tends to plug up with contaminants in the hydraulic fluid during operation.